Persistence of Eutrophicated Vegetation on Banks of Recharge Ponds in Dune Area Meijendel, the Netherlands

Abstract

In Meijendel, a coastal dune area used for drinking water production, the supply of nutrient rich river water to recharge aquifers since 1955 has resulted in eutrophication of the vegetation. Despite phosphate stripping of the recharge water since 1975, eutrophication has persisted, whereas the management target is a more biodiverse mesotrophic vegetation. This thesis explores possible causes of the persistent eutrophication, focussing on the complex interactions between nutrients, vegetation, and hydrology. As a measure of eutrophication, the mean N-Ellenberg value (N-value) of the bank vegetation of two recharge
ponds was determined. Also orthophosphate and nitrate concentrations of soil, groundwater and nearby surface water and hydrological parameters were measured and relationships between these factors statistically analysed. The N-value is associated with nutrient concentrations in soil and groundwater, orthophosphate in surface water, and the distance to the recharge water inlet. The concentration of orthophosphate in soil and groundwater is associated with the groundwater flow rate. Although no direct relationship has been
demonstrated between mineralisation and N-value, its influence cannot be ruled out either. On lower banks surface water influence certainly predominates, on higher banks mineralisation could play a (stronger) role. Remarkably, concentrations of orthophosphate in soil and water were below target values for a similar habitat type after restoration. It appears that nutrient concentrations in soil and groundwater can be relatively low in the persistence of eutrophic vegetation, lower than concentrations for establishing such vegetation composition. The difference in N-value between the two ponds could be explained by a difference in the amount of sludge in the ponds which may absorb orthophosphate in winter and release it early in spring. To regain a more diverse dune specific vegetation, a complete reset is necessary, by removing the current vegetation and the upper (acidified and organic matter enriched) soil layer down to the calcareous sand. Regular sludge removal of the bottom of
the recharge ponds may prevent accumulation of orthophosphate. More research is needed on the exact mechanisms behind the persistence of eutrophic vegetation at low nutrient concentrations. The roles and influence of nutrient loads and - fluxes, acidity, humidity, phosphate binding and release, and vegetation composition itself, are not yet sufficiently understood.

Date of Award	31 Mar 2023
Original language	English
Supervisor	J de Kraker (Supervisor), Dennis Uit de Weerd (Assessor) & H.G.J.M. van der Hagen (Supervisor)